Redundant control method for a polyphase converter with distributed energy stores

ABSTRACT

A method for controlling an inverter having at least two phase modules having respective upper and lower valve branches with at least three two-pole subsystems connected in series, includes in the event of failure of at least one subsystem in a faulty valve branch of a defective phase module the following method steps: a) identifying the faulty a defective upper or lower valve branch in the identified defective phase module in which one or more subsystems have failed; b) controlling a terminal voltage of the one or more failed subsystems in the faulty valve branch so as to be permanently zero; and c) controlling in each of the upper and lower fault-free valve branches having fault-free subsystems a number of fault-free subsystems corresponding to the one or more failed subsystems such that their terminal voltages of the controlled fault-free subsystems terminal voltages are permanently zero.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is the U.S. National Stage of International ApplicationNo. PCT/EP2009/055812, filed May 14, 2009, which designated the UnitedStates and has been published as International Publication No. WO2010/015431 and which claims the priority of German Patent Application,Serial No. 10 2008 036 810.5, filed Aug. 7, 2008, pursuant to 35 U.S.C.119(a)-(d).

BACKGROUND OF THE INVENTION

The invention relates to a method for controlling a converter withdistributed energy stores.

DE 101 03 031 A1 discloses a converter with distributed energy stores.An equivalent circuit of a converter such as this is shown in moredetail in FIG. 1. According to this equivalent circuit, this knownconverter has three phase modules, which are each annotated 100. Thesephase modules 100 are each electrically conductively connected on the DCvoltage side to a positive and a negative DC voltage busbar P₀ and N₀.In the case of a voltage intermediate-circuit converter, a seriescircuit of two capacitors C1 and C2, across which a DC voltage U_(d) isdropped, would be connected between these two DC voltage busbars P₀ andN₀. A connection point between these two capacitors C1 and C2, which areelectrically connected in series, forms a virtual neutral point O. Eachphase module 100 which forms a bridge arm of the polyphase converter hasan upper and a lower bridge arm element, which are referred to in thefollowing text as the respective valve arms T1, T3 and T5 as well as T2,T4 and T6, since the bridge arm elements each represent a convertervalve of the polyphase converter with distributed energy stores. Each ofthese valve arms T1 to T6 has a number of two-pole subsystems 10 whichare electrically connected in series. In this equivalent circuit, fourof these subsystems 10 are shown. The number of subsystems 10 per valvearm T1, . . . , T6 is, however, not restricted to this illustratednumber. Each junction point between two valve arms T1 and T2; T3 and T4as well as T5 and T6 of a phase module 100 forms a respective connectionL1, L2 and L3 on the AC voltage side of a phase module 100. Since, inthis illustration, the converter has three phase modules 100, athree-phase load, for example a polyphase motor, can be connected totheir connections L1, L2 and L3 on the AC voltage side, also referred toas load connections.

FIG. 2 shows an equivalent circuit of one known embodiment of a two-polesubsystem 10 in more detail. The circuit arrangement shown in FIG. 3represents a functionally completely equivalent variant. Bothembodiments of a two-pole subsystem 10 are known from DE 101 03 031 A1.These known two-pole subsystems 10 each have two semiconductor switches1 and 3 which can be turned off, in each case two diodes 2 and 4 and ineach case one unipolar energy storage capacitor 9. The two semiconductorswitches 1 and 3 which can be turned off are electrically connected inseries, with this series circuit being connected electrically inparallel with the energy storage capacitor 9. One of the two diodes 2and 4 is electrically connected in parallel with each semiconductorswitch 1 and 3 which can be turned off such that these diodes 2 and 4are connected back-to-back in parallel with the correspondingsemiconductor switch 1 or 3 which can be turned off. The unipolar energystorage capacitor 9 in the subsystem 10 consists either of a capacitoror of a capacitor bank composed of a plurality of such capacitors havinga resulting capacitance C₀. The connection point between the emitter ofthe semiconductor switch 1 which can be turned off and the anode of thediode 2 forms a connecting terminal X1 of the subsystem 10. Theconnection point between the two semiconductor switches 1 and 3 whichcan be turned off and the two diodes 2 and 4 forms a second connectingterminal X2 of the subsystem 10.

In the embodiment of the two-pole subsystem 10 shown in FIG. 3, thisconnection point forms the first connecting terminal X1. The connectionpoint between the collector of the semiconductor switch 1 which can beturned off and the cathode of the diode 2 forms the second connectingterminal X2 of the subsystem 10.

In both illustrations of the two embodiments of the two-pole subsystem10, Insulated Gate Bipolar Transistors (IGBT) are used as thesemiconductor switches 1 and 3 which can be turned off as shown in FIGS.2 and 3. It is likewise possible to use MOS Field-Effect Transistors,also referred to as MOSFETs. It is also possible to use Gate Turn OffThyristors, also referred as GTO thyristors, or Integrated GateCommutated Thyristors (IGCT).

According to DE 101 03 031 A1, the two-pole subsystems 10 in each phasemodule 100 of the converter as shown in FIG. 1 are switched to aswitching state I, II and III. In the switching state I, thesemiconductor switch 1 which can be turned off is switched on, and thesemiconductor switch 3 which can be turned off is switched off. Theterminal voltage U_(X21) which is present at the connecting terminals X1and X2 in the two-pole subsystem 10 is therefore equal to zero. In theswitching state II, the semiconductor switch 1 which can be turned offis switched off, and the semiconductor switch 3 which can be turned offis switched on. In this switching state II, the terminal voltage U_(X21)which is present is equal to the capacitor voltage U_(C) across theenergy storage capacitor 9. In the switching state III, bothsemiconductor switches 1 and 3 which can be turned off are switched off,and the capacitor voltage U_(C) across the energy storage capacitor 9 isconstant.

In order to allow this converter to operate in redundant form withdistributed energy stores 9 as shown in FIG. 1, it is necessary toensure that a faulty subsystem 10 is permanently short-circuited at itsterminals X1 and X2. This means that the terminal voltage U_(X21) of thefaulty subsystem 10 is zero irrespective of the current directionthrough the terminals X1 and X2.

The failure of one of the semiconductor switches 1 and 3 which can beturned off and are present in the subsystem 10, or of an associatedcontrol circuit, results in this subsystem 10 not operating correctly.Further possible causes of malfunctions are, inter alia, faults in theassociated control circuit for the semiconductor switches, their powersupply, communication and measured-value detection. That is to say, thetwo-pole subsystem 10 can no longer be controlled as desired in one ofthe possible switching states I, II or III. The short-circuiting of thesubsystem 10 at its connections X1 and X2 means that no more power issupplied to this subsystem 10. This reliably precludes consequentialdamage such as overheating and fire resulting from continued operationof the converter.

A conductive connection like a short circuit such as this between theconnecting terminals X1 and X2 of a faulty two-pole subsystem 10 has toreliably carry at least the operating current of one valve arm T1, . . ., T6 in the phase module 100 in which the faulty two-pole subsystem 10is connected, without overheating. DE 10 2005 040 543 A1 discloses how afaulty subsystem 10 can be reliably short-circuited in order that thisknown converter with distributed energy stores can continue to beoperated in a redundant form.

The following explanation is based on the assumption that the energystorage capacitors 9 in all of the two-pole subsystems 10 each have thesame voltage U_(C). Methods for initially producing this state and formaintaining it during operation are likewise known from DE 101 03 031A1. FIG. 4 shows a graph of a profile of the potential difference U_(PL)between the terminal P of a phase module 100 and a grid connection L,plotted against the time t. FIG. 5 shows a graph of a profile of thepotential difference U_(LN) between the terminal L and the potential atthe terminal N, plotted against the time t. According to these potentialprofiles U_(PL) and U_(LN), in each case one subsystem of the eighttwo-pole subsystems 10 of the valve arms T1 and T2 is connected ordisconnected at each of the times t1, t2, t3, t4, t5, t6, t7 and t8.Switching on in this case corresponds to a change from the switchingstate I to the switching state II. Turning off corresponds to a changefrom the switching state II to the switching state I. These two graphseach show one period T_(P) of a fundamental oscillation of the potentialprofile u_(L0) (FIG. 6) between the load connection L and the virtualneutral point O of a phase module 100 of the converter with distributedenergy stores 9, for the potential profiles U_(PL) and U_(LN).

FIG. 6 shows a profile of the difference between the potential profilesU_(LN) and U_(PL) as shown in FIGS. 4 and 5, in the form of a graphplotted against the time t. This resultant potential profile U_(LO)occurs between a connection L1, L2 or L3 on the AC voltage side of aphase module 100 in the converter with distributed energy stores 9 asshown in FIG. 1 and an arbitrarily selected potential of a virtualneutral point O of a voltage intermediate circuit having two capacitorsC1 and C2. Corresponding components of harmonics or DC voltagecomponents in each of the output voltages U_(LO) of the phase modules100 in the polyphase converter with distributed energy stores 9 as shownin FIG. 1 are resolved in the case of a balanced polyphase voltagesystem in the difference voltages between in each case two phase-shiftedoutput voltages U_(L10), U_(L20) or U_(L30). These two potentialprofiles U_(PL) and U_(LN) likewise show that the sum of the potentialsat any time is 4·U_(C). This means that the value of the DC voltageU_(d) between the DC voltage busbars P₀ and N₀ always corresponds to aconstant number of subsystems 10 in the switching state II multiplied bythe value of the capacitor voltage U_(C) across the capacitor 9. In thesituation illustrated by way of example, this number corresponds to thenumber of two-pole subsystems 10 in the valve arms T1, . . . , T6 in theconverter as shown in FIG. 1.

DE 10 2005 045 091 A1 discloses a method for controlling a converterwith distributed energy stores as shown in FIG. 1, by means of which thebalance conditions are maintained in the event of a malfunction of atleast one subsystem in a phase module of this converter. According tothis known method, one valve arm of one of the three phases in which oneor more of the two-pole subsystems is or are faulty is first of alldetermined. Each faulty subsystem is controlled such that the amplitudeof the terminal voltage is in each case zero. A number of subsystemscorresponding to the number of determined two-pole subsystems in afurther valve arm of the faulty phase module are controlled such thatthe amplitude of the terminal voltage is in each case equal to acapacitor voltage. This control of the subsystems in the faulty phasemodule is likewise carried out in subsystems in the valve arms of thesound phase modules.

FIG. 7 shows a graph of a profile of the potential difference U_(PL1)between the terminal P in a phase module 100 and a load connection L ina phase module 100, plotted against the time t, with one faulty two-polesubsystem 10 in the lower valve arm T2 and T4 and T6 in a phase module100. FIG. 8 shows a graph of a profile of the potential differenceU_(L1N) between the terminal L and the potential of the terminal N,plotted against the time t. As can be seen from the profile of thepotential difference U_(PL) in FIG. 7, a subsystem 10 in each uppervalve arm T1 and T3 and T5 of each phase module 100 is controlled suchthat its terminal voltage U_(X21) is always equal to the capacitorvoltage U_(C) across the energy storage capacitor 9. Of the foursubsystems 10 illustrated by way of example in each upper valve arm T1and T3 and T5, there are now only three remaining subsystems 10 whichare connected and disconnected. As can be seen from the time profile ofthe potential difference U_(LN) of each lower valve arm T2 and T4 and T6in each phase module 100, in each case one of the four subsystems 10illustrated by way of example is controlled such that its terminalvoltage U_(X21) is always equal to zero. As shown in FIG. 1, of theselower valve arms T2, T4 and T6 in the three phase modules 100, the valvearm T2 has a faulty two-pole subsystem 10, identified by shading. Thevalue of the amplitudes of the voltage U_(LN) of each valve arm T2, T4and T6 can therefore now be only at most 3·U_(C). This known methodresults in the number of subsystems 10 used in the event of a faultbeing equal to the number of subsystems 10 used when no faults arepresent. The profile of the amplitude of the sum of the potentialdifferences U_(PL) and U_(LN) is shown by means of a dashed line in thegraph in FIG. 8. In comparison to the situation when there are nofaults, the voltages U_(L10), U_(L20) and U_(L30) each have a lowermaximum amplitude when a fault is present. In the illustrated example,these voltages U_(L10), U_(L20) and U_(L30) when no fault is presenthave a maximum voltage amplitude of ½·U_(d) each, while in contrast themaximum amplitude when a fault is present is only ⅜·U_(d). This meansthat this known method results in a balanced three-phase voltage system,with a lower maximum amplitude, when a fault is present.

FIG. 9 shows a profile of the difference in the voltage differencesU_(PL) and U_(LN) as shown in FIGS. 7 and 8, plotted against the time t.As can be seen from this time profile of the potential between the loadconnection L1 or L2 or L3 and a virtual neutral point O, this potentialno longer oscillates symmetrically about a null position. This nullposition is shifted through ⅛·U_(d). This means that this potentialprofile has a DC component.

SUMMARY OF THE INVENTION

The invention is now based on the object of developing the known controlmethod such that no DC voltage components any longer occur in the outputvoltages of the converter with distributed energy stores when a faultoccurs.

This object is achieved according to the invention by a method forcontrolling an inverter having at least two phase modules which have anupper and a lower valve branch, which each have at least three two-polesubsystems, which are connected in series, in the event of failure of atleast one subsystem of a valve branch, wherein the valve branch with thefailed subsystem is determined, and wherein one subsystem of a valvebranch which corresponds to the failed valve branch in each fault-freephase module is in each case controlled such that its terminal voltagesare in each case zero, wherein one subsystem of a valve branch whichcorresponds to the failed valve branch in the failed phase module iscontrolled such that its output voltage is equal to zero, and such thatone subsystem of a valve branch which corresponds to this valve branchin each fault-free phase module is in each case controlled such that itsterminal voltages are equal to zero.

Since a number of two-pole subsystems, with this number corresponding tothe number of faulty subsystems, are controlled in a valve arm of afaulty phase module corresponding to the faulty valve arm, such that theamplitudes of their terminal voltages are equal to zero, the outputvoltage of the faulty phase module no longer has a DC component. Becauseof the symmetry condition, corresponding subsystems in the valve arms ofthe sound phase modules are controlled in a corresponding manner. Thistherefore results in a three-phase balanced voltage system with no DCvoltage component.

BRIEF DESCRIPTION OF THE DRAWING

In order to explain the invention further, reference is made to thedrawing, which schematically illustrates one embodiment of a methodaccording to the invention for controlling a multi-phase converter withdistributed energy stores.

FIG. 1 shows an equivalent circuit of a known converter with distributedenergy stores,

FIG. 2 shows an equivalent circuit of a first embodiment of a knowntwo-pole subsystem in the converter shown in FIG. 1.

FIG. 3 shows an equivalent circuit of a second embodiment of a knowntwo-pole subsystem in the converter as shown in FIG. 1.

FIGS. 4-6 show potential profiles of a phase module of a converter asshown in FIG. 1 when no faults are present, in each case in the form ofa graph plotted against the time t,

FIGS. 7-9 show potential profiles of a phase module in a converter asshown in FIG. 1 when a fault is present, in each case in the form of agraph plotted against the time t, and

FIGS. 10-12 illustrate voltage profiles of a faulty phase module in aconverter as shown in FIG. 1, in each case in the form of a graphplotted against the time t, generated by means of the method accordingto the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

It is now assumed that one two-pole subsystem 10 in the valve arm T2 ofthe phase module 100 in the converter with distributed energy stores 9as shown in FIG. 1 has been safely short-circuited because of somefault. This faulty two-pole subsystem 10 is indicated by means ofshading in the equivalent circuit shown in FIG. 1.

According to the method according to the invention, this faulty two-polesubsystem 10 must first of all be determined. Once this faulty subsystem10 has been determined, this subsystem 10 is controlled such that theamplitude of the associated terminal voltage U_(X21) is zero. This phasemodule 100, in which the valve arm T2 has a faulty subsystem 10, isreferred to in the following text as the faulty phase module 100.Furthermore, this faulty phase module 100 has a valve arm T1 in whichthere is no faulty subsystem 10. According to the method according tothe invention, a corresponding number of two-pole subsystems 10 in thesound valve arm T1 of this faulty phase module 100, which numbercorresponds to the number of faulty subsystems 10 in the faulty valvearm T2, are controlled such that the amplitude of the terminal voltageU_(X21) is in each case equal to zero. Since, in this example, only onetwo-pole subsystem 10 in the valve arm T2 is faulty, only one two-polesubsystem 10 is controlled in the associated valve arm T1 of the faultyphase module 100 such that the amplitude of its terminal voltage U_(X21)is equal to zero.

FIG. 10 shows a diagram plotted against the time t of the time profileof the potential difference U_(PL) at the terminal P with respect to aload connection L1. FIG. 11 shows a diagram plotted against the time tof the time profile of the potential difference U_(LN) of the terminal Lwith respect to the potential of the terminal N. As can be seen from thetwo potential profiles U_(PL) and U_(LN), only three subsystems 10 ofthe four two-pole subsystems 10 in the valve arms T1 and T2 areavailable for control purposes. The sum of these two potential profilesU_(PL) and U_(LN) once again results in a DC voltage U_(d), which ispresent between the DC voltage busbars P₀ and N₀ of this converter withdistributed energy stores 9 as shown in FIG. 1. This means that the DCvoltage U_(d) is the same whether or not there are any faults. Forbalance reasons, the two-pole subsystems 10 in the valve arms T4, T3 andT6, T5 of the two sound phase modules 100 in the converter withdistributed energy stores 9 as shown in FIG. 1 are controlled in acorresponding manner. This means that a number of subsystems 10, whichnumber corresponds to the number of faulty subsystems 10, are controlledin the sound valve arms T4 and T6 of the sound phase modules 100 of theconverter as shown in FIG. 1, which valve arms T4 and T6 correspond tothe faulty valve arm T2 in the faulty phase module 100, are controlledsuch that the amplitudes of their terminal voltages U_(X21) are eachzero. Since only one subsystem 10 is faulty in the faulty valve arm T2of the faulty phase module 100, only one two-pole subsystem 10 is ineach case controlled in valve arms T4 and T6, which correspond to thevalve arm T2, in the sound phase module 100 of the converter withdistributed energy stores 9 as shown in FIG. 1, such that the amplitudesof the terminal voltages U_(X21) are in each case zero. One subsystem 10in the faulty phase module 100 in the sound valve arm T1 is likewisecontrolled such that the amplitude of the associated terminal voltageU_(X21) is zero. This means that the number of faulty subsystems 10 inthe faulty valve arm T2 in the sound phase modules 100 of the converterwith distributed energy stores 9 as shown in FIG. 1 are controlled inthe valve arms T3 and T5, which correspond to the sound valve arm T1 ofthe faulty phase module 100, in the sound phase modules 100 ofsubsystems 10 in each case, such that the amplitudes of their terminalvoltages U_(X21) are likewise equal to zero.

Such control of two-pole subsystems 10 in the converter with distributedenergy stores 9 results in output voltages U_(L10), U_(L20) and U_(L30),which are respectively present between a connection L1, L2 and L3 on theAC voltage side, and a virtual neutral point O. These output voltagesU_(L10), U_(L20) and U_(L30) have a potential profile U_(L0) which isillustrated in a diagram plotted against the time t in FIG. 12. Thisprofile no longer has any DC component. The amplitudes of these outputvoltages U_(L10), U_(L20) and U_(L30) are respectively less than theamplitudes of the output voltages which have been generated by means ofthe known control method. According to the example of four subsystems 10per valve arm T1, . . . , T6, the output voltages U_(L10), U_(L20) eachhave an amplitude of ¼·U_(d) in comparison to an amplitude of ⅜·U_(d)(known control method). Instead this balanced three-phase voltage systemwith a lower amplitude has no DC voltage component.

The DC component which occurs in the output voltages U_(L10), U_(L20)and U_(L30) of the converter with distributed energy stores 9 in theknown method, as shown in FIG. 1, causes a shift in the starpoint in aconnected rotating-field machine, which can lead to bearing currents.Furthermore, when using the converter as an active, direct power supplysystem feed, said DC component causes a shift in the potential of theconverter with respect to ground potential when the starpoint on thepower supply system side is grounded. In some circumstances, thisrequires additional complexity for the insulation of the converter. Themethod according to the invention overcomes this disadvantage, althoughit must be accepted that the amplitudes of the output voltages U_(L10),U_(L20) and U_(L30) of the converter as shown in FIG. 1 will be lower.The greater the number of two-pole subsystems 10 which are used in thevalve arms T1, . . . , T6, the more finely graduated are the outputvoltages U_(L10), U_(L20) and U_(L30) of the converter with distributedenergy stores 9 as shown in FIG. 1. This makes it possible toapproximate to a sinusoidal profile even with faulty two-pole subsystems10.

The invention claimed is:
 1. A method for controlling an inverter havingat least two phase modules (100), with each phase module (100) having anupper valve branch (T1, T3, T5) and a lower valve branch (T2, T4, T6),and with each valve branch (T1 . . . T6) having at least three two-polesubsystems (10) connected in series, with the following method steps inthe event Of failure of at least one subsystem (10-1-2) in a fault˜valvebranch (T2) of a defective phase module (101): a) identifying the faultyvalve branch (T2) in the identified defective phase module (101) inwhich the at least one subsystem (10T2) has failed; b) controlling aterminal voltage of the at least one failed subsystem (10T2) in thefaulty valve branch (T2) so as to be permanently zero; and c)controlling each of the upper and lower fault-free valve branches (T1,T3 . . . T6) a number of fault-free subsystems (10T1, 10T3 . . . 10T6)corresponding to the at least one failed subsystem (10T2) such thatterminal voltages of the controlled fault-free subsystems (10T1, 10T3—.10-r6) are also permanently zero.
 2. A method for controlling aninverter having at least two phase modules, with each phase modulehaving an upper valve branch and a lower valve branch, and with eachvalve branch having at least three two-pole subsystems connected inseries, with the following method steps in the event of failure of atleast one subsystem in a faulty valve branch of a defective phasemodule: a) identifying the faulty valve branch in the identifieddefective phase module in which one or more subsystems have failed; b)controlling a terminal voltage of the one or more failed subsystems inthe faulty valve branch so as to be permanently zero; and c) controllingeach of the upper and lower fault-free valve branches having fault-freesubsystems; a number of fault-free subsystems corresponding to the oneor more failed subsystems such that terminal voltages of the controlledfault-free subsystems are permanently zero.